1. Field of the Invention
This invention relates to a method for concurrently preparing dimethylchlorosilane and a triorganochlorosilane.
2. Prior Art
Dimethylchlorosilane is a monomeric source material playing an important role in the silicone industry. For example, it is used as a starting material for preparing silicone fluides and silicone resins having a Si-H bond at one end or in a backbone thereof. It is also used as an intermediate in the synthesis of organic silicon compounds such as silylating agents.
On the other hand, triorganochlorosilanes, for example, trimethylchlorosilane and derivatives thereof are not only used in a wide range of application as silylating agents, but also effective for treating inorganic materials to be hydrophobic and introducing a terminal block unit into an organopolysiloxane chain. Also t-butyldimethylchlorosilane is important as a silylating agent for the synthesis of medicines such as antibiotics and other organic compounds.
Several methods are known in the prior art for the preparation of dimethylchlorosilane. For example, when dimethyldichlorosilane is prepared by a direct process, by-products are obtained along with hydrocarbons in a low-boiling component. Dimethylchlorosilane is isolated from the by-products by distillation. This process has the drawback that dimethylchlorosilane is obtained only in a small yield despite a relatively long process time. Alternatively, dimethylchlorosilane is prepared by starting with tetramethyldisiloxane. Since tetramethyldisiloxane is prepared by cracking an oil containing a Si-H bond, the overall process is time consuming and entails the risk of cleavage of a Si-H bond during cracking.
It is reported in Kogyo Kagaku Zassi (Journal of Industrial Science), 60, 1395 (1957) to synthesize dimethylchlorosilane by reacting pentamethylchlorodisilane with dry hydrogen chloride at high temperature. Chem. Comm., 507 (1970) reports another synthesis route involving passing hydrogen chloride through polydimethylsilane under exposure of UV radiation to thereby react polydimethylsilane with hydrogen chloride. These methods use expensive reactants and are not generally acceptable in the industry.
JP-A 53-95922 discloses a method for preparing a dialkylmonochlorosilane by reducing a dialkyldichlorosilane with sodium borohydride and optionally sodium hydride in N,N,N',N',N",N"-hexamethylphosphortriamide. However, since N,N,N',N',N",N"-hexamethylphosphortriamide was recently found strongly carcinogenic, this method was commercially prohibited from use.
A still further method is reduction of a silicon-chlorine bond in a chlorosilane using a lithium aluminum hydride. When a dialkyldichlorosilane is reduced with such a reducing agent, both of the two chlorine atoms undergo reduction, resulting in a dialkylsilane, but not a dialkylmonochlorosilane.
For preparing trimethylchlorosilane, several methods are known in the art other than the direct method. Commonly employed is a synthetic method based on reaction involving dimethyldichlorosilane, methyltrichlorosilane or tetrachlorosilane using a Grignard reagent. This method is difficult to selectively produce only trimethylchlorosilane while a relatively large amount of tetramethylsilane is by-produced. This requires a cumbersome step of separating and purifying trimethylchlorosilane from concomitant methylsilanes at the end of reaction. In Z, Anorg. Allgem. Chem., 287, 273 (1956), an attempt was made to effect methylation reaction of tetrachlorosilane or methyltrichlorosilane using methylaluminumsesquichloride. This results in a mixture of various methylsilanes, failing to selectively produce trimethylchlorosilane as a single component.
JP-B 57-30114 discloses that trimethylchlorosilane is obtained in a compositional ratio of 81.7% by passing methylhydrogendichlorosilane and methyl chloride through metallic aluminum at 180.degree. to 450.degree. C. However, this method requires high temperature and suffers from the serious risk that the by-produced aluminum chloride can clog the flow system. It is also known to prepare trimethylchlorosilane by passing hydrogen chloride gas through tetramethylsilane in the presence of a Friedel-Crafts catalyst (JP-A 56-92895) and by effecting disproportionation reaction between tetramethylsilane and dimethylchlorosilane in the presence of a Friedel-Crafts catalyst (JP-A 55-61195). Since a preceding step of synthesis reaction is required to prepare tetramethylsilane which is the starting reactant, the overall process is a combination of two steps and thus less efficient.
On the other hand, t-butyldimethylchlorosilane is synthesized by chlorinating t-butyldimethylsilane with chlorine. This process fails to effectively utilize hydrogen atoms because they are lost as hydrogen chloride.